This invention relates to improved bimodal resin compositions useful for making electrostatographic toner compositions, and processes for producing the same, More particularly, this invention relates to one-step anionic polymerization processes for producing bimodal resin compositions, In embodiments of the present invention, there are described anionic polymerization processes that can be used to produce toner compositions in an economical manner. The bimodal resin compositions described in this invention include homopolymers or copolymers polymerizable by anionic polymerization, such as styrene/butadiene resin compositions. More specifically, the bimodal resin compositions obtained by the anionic polymerization processes of the present invention are characterized by a narrow molecular weight distribution of the two macromolecular species in the resin composition.
Bimodal toner resins are known in the art to be useful in electrophotography and related imaging techniques, Such bimodal toner resins are preferred for formulating certain toner compositions because they allow more specific tailoring of fusing characteristics of the resin as compared to monomodal resins. Most conventional toner resins are monomodal, with a relatively narrow to broad molecular weight distribution. Resins with a broad molecular weight distribution, i.e., a high degree of polydispersity (Mw/Mn or MWD) are often beneficial for improving certain fusing characteristics, such as fusing latitude. In some cases, bimodal toner resins can provide for more advantageous fusing characteristics than the monomodal resins with broad molecular weight distribution.
Bimodal toner resins, including bimodal styrene/acrylate toner resins, are known in the art. Processes used for making such bimodal toner resins involve a multi-step process. For example, U.S. Pat. Nos. 5,219,946 and 5,219,947 disclose bimodal binder resins and processes for producing such resins. Both of these patents disclose a process for producing the binder resin wherein a first stage resin (a polymer or copolymer) is prepared by solution polymerization. The first stage resin is then dissolved in a polymerizable monomer. In a second stage of the process, the polymerizable monomer is subjected to suspension polymerization in the presence of the first-stage resin and a cross-linking agent. The patents also disclose that such bimodal toner resins may be produced by thermo-mechanical mixing of two polymers having different molecular weights, although that process is not as preferred as the two-stage solution and suspension polymerization process. The molecular weight distribution of the binder resins produced by the process of the cited patents has a first peak in the region of molecular weight of from 2,000 to 10,000 and a second peak or shoulder in the region of molecular weight of from 15,000 to 100,000. The binder resins also have a molecular weight distribution (MWD) of greater than 5 and less than 80.
Processes for producing toner resins by anionic polymerization processes are also known in the art. For example, U.S. Pat. Nos. 5,158,851 and 5,215,846 disclose anionic polymerization processes for producing toner resins comprised of multiblock or liquid glass resin particles with a glass transition temperature of from about 20.degree. C. to about 65.degree. C. The multiblock or liquid glass type toner resins of the latter patents include segments of polystyrene and segments of polybutadiene. U.S. Pat. No. 4,581,429 discloses that although anionic polymerization provides satisfactory results in some circumstances, the process is difficult to apply on an industrial scale. For example, it is disclosed that the stringent reaction conditions required by the anionic polymerization process are difficult to attain on an industrial scale. Furthermore, it is disclosed that many monomers cannot be polymerized by an anionic process.
U.S. Pat. Nos. 5,166,028 and 4,469,770 describe styrene/butadiene copolymer resins, methods for their manufacture, and their use in toners and developers. U.S. Pat. No. 5,166,028 discloses a monomodal styrene/butadiene toner resin composition having a molecular weight of from about 10,000 to about 500,000 and a molecular weight distribution of from about 5 to about 120. The monomodal styrene/butadiene resin is produced in a dispersion polymerization process. U.S. Pat. No. 4,469,770 discloses a monomodal styrene/butadiene copolymer resin composition having a molecular weight of from about 45,000 to about 155,000 and a molecular weight distribution of about 7. The monomodal styrene/butadiene resin composition is produced in an emulsion polymerization process.
Although bimodal toner resins are known in the art, there continues to be a need for improved toner resin compositions, especially bimodal toner resin compositions with enhanced functional properties. Such bimodal toner resins are preferred for formulating toner compositions because they allow more specific tailoring of fusing characteristics of the resin as compared to monomodal resins. Thus, bimodal toner resins with more narrow molecular weight distributions allow for more advantageous fusing characteristics.
A need also continues to exist in the art for improved methods for producing such bimodal toner resins. Although methods are currently known for producing bimodal resins, such methods typically have inherent drawbacks that hinder their application in commercial uses. For example, the method of thermo-mechanical mixing of two polymers having different molecular weights typically results in an inhomogeneous mixture having less desirable fusing characteristics. As a further example, the two-stage polymerization process described above requires additional equipment and processing as compared to production methods for monomodal toner resins, and therefore increases the cost of producing bimodal resins. For example, Arthur S. Diamond and Lynn S. Ritter, "Toner in the '90S: The Shape Of The Things To Come," pp. 121-122 disclosed that the price for a styrene/acrylate bimodal resin in January, 1989, was about $1.75 per pound, as compared to $1.25 per pound for a conventional (monomodal) resin. The cost of the bimodal resin is thus 40% higher than for the comparable monomodal resin.
Anionic polymerization processes are known to require more stringent purity and reaction conditions. Nevertheless, there is an established commercial styrene/butadiene and styrene/isoprene anionic polymer market with prices of the polymers close to the range of other commodity polymers. A need continues to exist, however, for improved anionic polymerization processes for special applications, such as toner resins.